Coots use hatch order to learn to recognize and reject conspecific brood parasitic chicks.

Avian brood parasites and their hosts provide model systems for investigating links between recognition, learning, and their fitness consequences. One major evolutionary puzzle has continued to capture the attention of naturalists for centuries: why do hosts of brood parasites generally fail to recognize parasitic offspring after they have hatched from the egg, even when the host and parasitic chicks differ to almost comic degrees? One prominent theory to explain this pattern proposes that the costs of mistakenly learning to recognize the wrong offspring make recognition maladaptive. Here we show that American coots, Fulica americana, can recognize and reject parasitic chicks in their brood by using learned cues, despite the fact that the hosts and the brood parasites are of the same species. A series of chick cross-fostering experiments confirm that coots use first-hatched chicks in a brood as referents to learn to recognize their own chicks and then discriminate against later-hatched parasitic chicks in the same brood. When experimentally provided with the wrong reference chicks, coots can be induced to discriminate against their own offspring, confirming that the learning errors proposed by theory can exist. However, learning based on hatching order is reliable in naturally parasitized coot nests because host eggs hatch predictably ahead of parasite eggs. Conversely, a lack of reliable information may help to explain why the evolution of chick recognition is not more common in hosts of most interspecific brood parasites.